oil found on spark plug?

i pulled my #1 spark plug and it keeps fouling from oil,car runs fine untill the plug fouls{about 2 weeks} any suggestions?
can a real bad leaking valve cover cause this?dont have the money to rebuild yet,and like i said the car runs great and doesnt use much oil and doesnt smoke till the plug fouls out, anyway to get longer life from the plug?if it requires a valve job what am looking roughly on the cost?the car is a 85 190E
thanks steve